To be able to produce very thin substrates, in particular semiconductor substrates such as wafers with a thickness of a few nm up to 250 μm, the latter are usually mounted on carrier systems. Starting from a thick (usually greater than 250 μm) substrate, in most cases a carrier foil is laminated on one side (so-called “back-thinning tape”) in order to be able to subsequently back-thin the substrate.
The back-thinning of wafers is often necessary in the semiconductor industry and can be implemented mechanically and/or chemically. For back-thinning, the wafers are generally temporarily attached to a carrier, whereby there are various methods for attachment. As carrier material, for example, foils, glass substrates or silicon wafers are used. At the end of the back-thinning process and the reworking within a unit, the back-thinned substrates are mounted on film frames, and then the carrier material is removed.
As soon as a working of the substrate that goes beyond the back-thinning is necessary, rigid carrier systems are used. Examples of such working steps on processing units after back-thinning are: metallization, dry etching, wet etching, laser processing, lithography, doping, etc.
In rigid carrier systems, the substrate that is to be worked is connected to a carrier substrate by an adhesive layer. The carrier substrate is typically between 250 and 1,500 μm thick.
The carrier substrate is to impart adequate mechanical stability to the substrate to be worked to any thinness in order to be able to be worked in related process steps or process units. The selection of the carrier substrate as well as the adhesive depends on the requirements of the subsequent working steps. In particular, the maximum operating temperature, vacuum resistance, optical transparency, chemical resistance, as well as the capacity to offset rough spots are to be considered.
The purpose of such carrier systems consists in being able to further work the thin substrates on mechanically stable carrier substrates in standard units or standard processes, without damaging the thin substrates in the further working, or to develop costly units that are specifically set up for the thin substrates.
Even during transport of the thin substrates from one unit to the next, protection of the to some extent already worked substrates is important to prevent damage to the fragile substrates.
Some of the above-mentioned working steps require an exact positioning of the substrate or the carrier within the corresponding unit, for example under a microscope with high magnification. There, the substrate has to be positioned within one μm to make possible a quick further working/inspection. If the position of the substrate that is to be detected after the positioning is not in the field of vision of the microscope, a search routine or a readjustment is necessary, by which the productivity drops. The positioning is carried out, for example, on the outside contour of the substrate or carrier, for example a flattened surface (so-called “flat”) or a recess (so-called “notch”).
It is therefore often necessary that in the carrier systems, positioning of the substrate is performed based on the outside control of the carrier so that as exact a positioning of the substrate as possible on the carrier is desirable.
Since the outside contours of the substrates or carrier substrates are associated with to some extent considerable manufacturing tolerances, positioning with simple mechanical stops is not adequate for a μm-precision positioning. Very narrowly tolerated substrate dimensions were no longer economical with a considerable additional expenditure in the production. Also, application of optically detectable passmarks increases the cost of the carrier system and is not possible in all cases.